The present invention relates to a process for producing coated paper containers having high impermeability to liquid. More particularly, the present invention relates to a process for producing paper containers having high impermeability to liquid which comprises spray coating a polymerizable solution containing a prepolymer onto a wall surface of, for example, a paper container, irradiating the coated wall of the container with ultraviolet light from all directions to effect the setting of said prepolymer on the wall surface of the container, thus forming a coating which is impermeable to liquids, such as water, milk, softdrinks, oils, etc.
Handy paper containers have hitherto been used for temporarily storing liquids such as water. Such containers, however, generally have a disadvantage in that the liquid gradually penetrates the paper constituting the container to render the paper wet; this decreases the mechanical strength of the container and sometimes leads to leakage of the liquid contents through the wall of the container. Therefore, it has been a practice to treat the wall surface of the container so as to prevent the penetration of the liquid contents into the paper constituting the container. The wax coating is one example of such treatments. In this method, wax is coated on the surface of the container by spraying a melted wax onto the wall surface of the container by a spray gun or the like or by dipping the container in melted wax and draining off the excess wax and allowing the deposited wax to solidify on cooling to give a water resistant container. However, this method suffers from several disadvantages. For example, it is very difficult to uniformly coat the wax over the wall surface of the container. Also, wax is expensive, so use of wax raises the cost of the product. Further, a wax coating is not applicable to containers for storing oily liquids, which have affinity for the coated wax, i.e., the wax will be dissolved in the oil and as a result the oil will be stained. Therefore, the use of wax coated containers is usually limited to products having no affinity for wax such as water, aqueous solutions, etc. Furthermore, when wax coated containers are bent at a low temperature, the wax on the wall surface cracks or comes off upon contact with cutlery or other edged or pointed metalware. In addition, although wax-coated water proof containers can be used to hold a liquid at a relatively low temperature, they are not suitable for holding hot (e.g. 50.degree.-100.degree. C.) products, because the wax softens and melts. A further disadvantage is that wax must be melted at a high temperature and the wax vapor produced by melting the wax spreads throughout the workshop and may be injurious to the operators' health.
Besides the wax coating method, another treating method has been practiced. In this method, a paper stock from which such containers are to be fabricated is provided on one or both sides thereof with a film of thermoplastic material such as polyethylene. Container components i.e. container body blanks and bottom panels or walls having a predetermined shape are then subsequently cut from the sheet material and formed into, for example, cup-shaped, containers having a coating of plastic film on the wall surface by a conventional method. However, this method also has inherent drawbacks. One disadvantages is that when body blanks are formed about a round mandrel with a plastic film coating on, for example, the inside so as to bring opposite cut ends of the blank into slight overlapping relationship so as to form an adhesively bonded side seam extending in axial direction along the container side wall, bonding the cut ends with an adhesive is very difficult in axial direction of the container body due to the presence of the coated plastic film on either of the cut ends. Another disadvantage is that the inner edge forming the side seam is inherently an uncoated edge, so if it is left uncoated, the liquid contents of the container will gradually penetrate through the uncoated or raw edge into the paper, thus leading to swelling and weakening of the paper and decrease in strength of the container. Still another disadvantage is that if a bottom seam joining the container body wall and the bottom panel or wall contains voids in the adhesive used to effect the seal it may result in leaking of the contents of the container through the bottom seam. Furthermore, because of the difference in level along the inner edge of the side seam the water filled in the container ascends up to the mouth rim of the container by capillary action and overflows. Therefore, due to the necessity to minimize the area of the raw edge and bottom seam, this method is only applicable at best for the manufacture of generally tapered cylindrical containers and is not suitable for fabricating containers of complicated configuration such as rectangular, hexagonal prisms, etc. Because the containers are fabricated from the paper stock having a plastic coating, there is also a possibility that the plastic film exhibits whitening along the bend at the container mouth rim, bottom edge etc. or the film peals off the paper. The scrap remaining after the body blanks are cut from the sheet can be used for nothing, and must be thrown away, which is an undesirable waste of a valuable petroleum resource, because the scrap is also coated with the plastic film. Still another method being practiced at present is the lining method in which the interior wall surface of the container is lined with a thermoplastic film. In this method, the containers obtained look as if a plastic bag is interposed into the container so as to conform to the side wall and the bottom panel shapes. Thus, there is no fear that the contents of the container will penetrate into the paper and that the contents may leak out through the voids in the bottom seam. However, this method sometimes fails to give uniform thickness at the mouth rim or bottom part of the container. Moreover, since the plastic film lining is only applicable to the interior of the container, it is necessary to use this method in combination with the above mentioned method of forming a container from the blank having the plastic film laminated on the outer surface in order to obtain a container having liquid-impermeable coating on both sides. Yet still, the outer edge forming the side seam extending in an axial direction along the container side wall was an uncoated edge through which, if it were left uncoated, moisture would eventaully penetrate. Such being the case, there has not been presented a process for easily and economically producing paper containers having high liquid impermeability for a wide range of applications. Up to the present there has been no successful attempt to produce a paper container having no uncoated raw edge by a single, simple and speedy method.